Light valves based on liquid crystal display technology, as well as MEMS (Micro-electro-mechanical systems) technology, have been used in various systems and applications, which include but are not limited to projectors, projection TVs, camcorders, digital still cameras, internet appliances, cell phones and headsets. In most of light valve applications, low cost, compactness and lightweight of the illumination system is desired. In addition, a uniform, bright and stable image is an important requirement in such applications.
FIGS. 1A and 1B show examples of prior art straight 10 and tapered 20 light guide integrators, respectively, that increase light uniformity in various known illumination systems.
The straight light guide 10 can be a solid glass rod with polished surfaces, or alternatively, a hollow tunnel with reflective surfaces. The light enters the entrance aperture 1 and emerges from the exit aperture 2 more uniformly after experiencing multiple reflections, in case of hollow light tunnels, or multiple total internal reflections, in case of solid light rods. The light uniformity at the exit aperture 2 increases with the increase in the length L of the light guides 10, 20.
As shown in FIG. 1A, the entrance 1 and exit 2 apertures of the straight light guide 10 have equal W1×W2 cross-sectional areas.
The tapered light guide 20 of FIG. 1B delivers more uniform and collimated light in comparison with straight light guides of FIG. 1A. A tapered light guide 20 usually has unequal cross-sectional areas A1, A2 at its entrance and exit apertures 3,4. Entrance 3 and exit 4 apertures can have unequal sizes and similar aperture shapes such as square, rectangular or circular, as well as unequal sizes and different aperture shapes. Such light guides have been described in U.S. Pat. No. 6,332,688, to Magarill, U.S. Pat. No. 5,829,858 to Levis and U.S. Pat. No. 4,765,718, to Henkes.
Known light integrating technologies suffer from inefficiency in light coupling and lack of compactness. Therefore, there is a need for a compact, lightweight, efficient and cost-effective illumination system to provide uniform and collimated light over a predefined area, such as the active area of a light valve.